I recently embarked on a landscape lighting project and ended up going HOG WILD!!!! Now, I've got nearly 2.5 acres with a lot of large trees I wanted to uplight and did get two bids from "pros" although here in WI, we don't need ANY type of license for low voltage, which is good for me and DIY, but also means nearly ANYONE can get into that business.
I ended up DIY'ing it and was lucky enough to get some GREAT advice from several out of state pros, who judging from their websites, they were true pros.
All said the same thing-PIPE IT!!!!! Yes, proper landscape wire in direct burial rated, but its 100x better off fully protected in pipe. Also, if you go with any of the store brands, cut off those STUPID wire pinching connectors (why poke holes in elec wire insulation???) and you'll get far more years of use out of it!!